The Role of Additional Pumped Hydro Storage in a …Department of Mechanical and Aerospace Engineering The Role of Additional Pumped Hydro Storage in a Low Carbon UK Grid Author: Thomas

Department of Mechanical and Aerospace Engineering

The Role of Additional Pumped Hydro Storage in a

Low Carbon UK Grid

Author: Thomas Hoy

Supervisor: Dr Andrew Grant

A thesis submitted in partial fulfilment for the requirement of the degree

Master of Science

Sustainable Engineering: Renewable Energy Systems and the Environment

2015

Copyright Declaration

This thesis is the result of the author’s original research. It has been composed by the

author and has not been previously submitted for examination which has led to the

award of a degree.

The copyright of this thesis belongs to the author under the terms of the United

Kingdom Copyright Acts as qualified by University of Strathclyde Regulation 3.50.

Due acknowledgement must always be made of the use of any material contained in,

or derived from, this thesis.

Signed: Thomas Hoy Date: 03/09/201

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Abstract

Electricity from renewable sources in 2014 contributed to a fifth of the total energy

generated in the UK. There is a clear trend leading to larger percentages of renewables

in the total installed generating capacity of the national grid. As these numbers creep

higher, problems of balancing supply with demand and preventing curtailment of

valuable renewable energy will become increasingly difficult. One solution to this is

energy storage and currently the only grid level form of storage is pumped hydro.

Luckily for the UK, various appraisals have identified pumped hydro storage as an

under-developed resource with the Highlands of Scotland and much of Wales having

the right geography for expansion of this tried and tested energy storage technology.

Actual supply and demand data from 2014 was used to build a model that simulated

increases in the UK’s installed renewable and nuclear capacity, this allowed scenarios

to be created that would show how much energy storage would be needed with

various increases in renewable and nuclear capacity.

Key objectives were to arrive at a figure for the level of new pumped storage the UK

would need to build to prevent curtailment of renewable energy supplies and

minimise reliance on gas turbines for load following.

The results indicated that different mixes of generating capacity mandated different

storage levels, but that once renewables that were mainly comprised of wind (as is

likely to be the case in the UK) begin to contribute around 30% of total electricity

generated then the amount of spilled energy will increase markedly. However,

200GWh of storage capacity would be able to save the majority of this otherwise

curtailed energy. It was also discovered that storage size had little effect on the size of

backup capacity required in the event of renewables not generating enough energy to

meet demand.

The overall conclusion was that there was a very strong case for building between 150

and 200GWh of new pumped storage in the UK in the very near future. However

realistically as renewable penetration increases so too must backup capacity in the

form of combined cycle gas turbines.

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Acknowledgments

I would firstly like to thank my wife Seonad for providing emotional and financial

support during my return to higher education, I would not have been able to do this

without her.

I am also greatly indebted to my supervisor Andy Grant who made himself available

whenever I needed a meeting and was always very quick in his responses to my

numerous emails.

I would finally like to thank the University of Strathclyde and Dr Paul Strachan for

allowing me the opportunity to return to continue my education after such a long

break.

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Table of Contents

1. Introduction ....................................................................................................... 11

1.1. Background ................................................................................................... 11

1.2. Project Objectives ......................................................................................... 12

1.3. Project Scope ................................................................................................. 12

1.4. Methods ......................................................................................................... 13

2. Literature Review .............................................................................................. 13

2.1. Pumped Storage ............................................................................................. 13

2.1.1. Turbine Types ............................................................................................ 15

2.1.2. Reservoirs .................................................................................................. 17

2.1.3. Pumped Storage in the UK ........................................................................ 20

2.2. UK Renewable Intermittency and Variability ............................................... 22

2.2.1. Wind Power Output ................................................................................... 22

2.2.2. Solar Power Output ................................................................................... 27

2.2.3. Hydro Output ............................................................................................. 32

2.2.4. Nuclear Power Variability ......................................................................... 34

2.3. Grid Stability with Renewables..................................................................... 36

2.3.1. An overview of the Grid ............................................................................ 36

2.3.2. The Challenge of Variable Renewables .................................................... 45

2.4. Storage with Large Renewable Penetrations ................................................. 47

3. Modelling of Grid ............................................................................................. 48

3.1. Demand and Renewable Data Source ........................................................... 48

3.2. Solar Modelling ............................................................................................. 50

3.3. The 2014 Base Case ...................................................................................... 51

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3.3.1. Wind .......................................................................................................... 52

3.3.2. Nuclear ....................................................................................................... 53

3.3.3. Coal ............................................................................................................ 53

3.3.4. Conventional Hydro .................................................................................. 54

3.3.5. Biomass ..................................................................................................... 54

3.3.6. CCGT ......................................................................................................... 54

3.3.7. Other Supply Options ................................................................................ 55

3.3.8. Demand in 2014 ......................................................................................... 55

3.4. Weaknesses ................................................................................................... 55

3.5. Scenarios Investigated ................................................................................... 57

4. Results ............................................................................................................... 63

4.1. Scenarios 1-4 ................................................................................................. 63

4.2. Scenarios 5-8 ................................................................................................. 67

4.3. Scenario 9 ...................................................................................................... 70

4.4. Scenario 10 .................................................................................................... 72

4.5. Scenario 11 .................................................................................................... 74

4.6. Scenario 12 .................................................................................................... 75

4.7. Scenarios 13-15 ............................................................................................. 76

5. Discussion ......................................................................................................... 78

5.1. When Will Curtailment Start and Storage be Required? .............................. 78

5.2. How Much PHS Could be Built in the UK? ................................................. 80

5.3. What Scenario is best? .................................................................................. 80

5.4. How Much PHS Should be Built?................................................................. 82

5.5. Infrastructure Requirements to Store Surplus Renewable Energy ................ 82

5.6. What This Means for CCGT’s Role in the Grid ........................................... 84

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6. In Conclusion .................................................................................................... 85

7. Further Work ..................................................................................................... 85

8. Works Cited ...................................................................................................... 86

9. Appendix ........................................................................................................... 90

List of Figures

Figure 1 Capacity vs Discharge time of Energy Storage Technologies .................................. 11

Figure 2 Wind Resources Across Europe ................................................................................ 13

Figure 3 Diagram of a Standard Francis Turbine .................................................................... 15

Figure 4 Photograph of Large Francis Turbine ........................................................................ 16

Figure 5 Kaplan Turbine Diagram ........................................................................................... 17

Figure 6 Output of Single vs Distributed Windfarms .............................................................. 23

Figure 7 Wind Resources Across Europe ................................................................................ 24

Figure 8 UK Wind Power Output in 2013 ............................................................................... 25

Figure 9 Forecast vs Actual Wind Power Output .................................................................... 26

Figure 10 Annual Solar Output for Single 125W Panel .......................................................... 28

Figure 11 Daily Output from 250W Panel ............................................................................... 29

Figure 12 Solar Resource Across the UK ................................................................................ 30

Figure 13 UK Regional Population of Solar Panels ................................................................ 31

Figure 14 Application Split of Solar PV .................................................................................. 32

Figure 15 UK 2014 Hydropower Output ................................................................................. 33

Figure 16 UK 2014 Nuclear Power Output ............................................................................. 36

Figure 17 Typical Grid Layout ................................................................................................ 37

Figure 18 Electricity by Fuel Source 2014 .............................................................................. 38

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Figure 19 Type of Electricity Generated 1980-2014 ............................................................... 39

Figure 20 UK Electricity Transmission Network .................................................................... 39

Figure 21 Comparison of Grid Strength in Various Countries ................................................ 40

Figure 22 Typical UK Annual Demand Profile ....................................................................... 42

Figure 23 Typical UK Daily Demand Profile .......................................................................... 43

Figure 24 OCGT use in 2014 ................................................................................................... 45

Figure 25 Wind Power Output Winter 2014 ............................................................................ 52

Figure 26 Geographical Distribution of UK Wind Farms ....................................................... 56

Figure 27 Scenarios 1-4 Factor increases in Capacity ............................................................. 57

Figure 28 Scenarios 5-8 Factor increases in Capacity ............................................................. 58

Figure 29 Scenario 9 Factor increases in Capacity .................................................................. 59

Figure 30 Scenario 10 Factor increases in Capacity ................................................................ 60



Figure 33 Scenarios 13-15 Factor Increases in Capacity ......................................................... 62

Figure 34 Decrease in CCGT Use When Compared to having No Storage ............................ 64

Figure 35 250GWh PHS System Use Throughout Year ......................................................... 65

Figure 36 Scenario 3 Energy Store Level 14-17 November .................................................... 66

Figure 37 Decrease in CCGT Use Compared to Having No Storage ...................................... 68

Figure 38 Solar Energy vs Pumped Storage Energy 18th

March ............................................. 69

Figure 39 Decrease in CCGT Use at Different Wind Capacities ............................................ 71

Figure 40 Curtailed Wind Vs Curtailed ................................................................................... 71

Figure 41 Storage Required To Replace CCGT Use ............................................................... 77

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List of Tables

Table 1 Existing Pumped Hydro Storage in UK ...................................................................... 21

Table 2 Scenario 1 Results ...................................................................................................... 63

Table 3 Scenario 9 Results ....................................................................................................... 70

Table 4 Scenario 10 Results ..................................................................................................... 72



Table 7 Scenarios 13-15 Results .............................................................................................. 76

Table 8 Final Scenario Results ................................................................................................ 81

Table 9 Appendix: Scenario 1 Results .................................................................................... 90

Table 10 Appendix: Scenario 2 Results .................................................................................. 91








Table 18 Appendix: Scenario 10 Results ................................................................................ 99

Table 19 Appendix: Scenario 11 Results .............................................................................. 100

Table 20 Appendix: Scenario 12 Results .............................................................................. 101

Table 21 Appendix: Scenarios 13-15 Results ....................................................................... 101

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List of Abbreviations

PHS –Pumped Hydro Storage

CCGT –Combined Cycle Gas Turbines

OCGT –Open Cycle Gas Turbines

HVDC –High Voltage Direct Current

DUKES –Digest of United Kingdom Energy Statistics

GWh –Giga Watt Hours

OFGEM – Office of Gas and Electricity Markets

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1. Introduction

1.1. Background

Renewables are making up an ever larger proportion of the total installed generating

capacity of our national grid, as this trend continues the role of storage will possibly

change from one of a purely load balancing mechanism to one that also includes what

could be described as a supply balancing mechanism. This means currently pumped

hydro storage (PHS) primarily exists in the UK to help deal with surges and falls in

demand but in the future this may change to primarily deal with surges and falls in

supply as the percentage of our electricity from variable supplies increases.

Pumped hydro storage is the only grid level form of energy storage currently available

to us although battery, flywheel, compressed air and super capacitor technology

continue to mature they do not yet store energy in the way we require to power a grid

in terms of storage capacity or discharge times (Figure 1).

Figure 1 Capacity vs Discharge time of Energy Storage Technologies

(EIA.GOV, 2011)

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Luckily for the UK it has an abundance of the type of geography that PHS requires,

lots of high hanging valleys sitting above large bodies of water exist in both Scotland

and Wales.

As the variety and size of planned future renewable capacity is uncertain and affected

by the vagaries of both politics and economics knowing when and in what size energy

storage will be required is a hard value to quantify, this project aims to hopefully

answer some of these questions.

1.2. Project Objectives

To arrive at a figure for how much new pumped hydro storage the UK will

require under a variety of supply scenarios.

To find out how much reserve capacity must be kept to backup renewables

under these same scenarios.

To find out at what level of installed renewables it will become desirable to

have additional energy storage built into the national grid.

1.3. Project Scope

The project will focus specifically on how much storage will be required under

various scenarios that are designed on the premise that the UK’s national grid is cut

off from all other countries, imports/exports of energy will not be considered as the

future make-up of foreign electrical grids and their demand/ supply characteristics

would be difficult to model.

Future demand side changes were left out also to keep the number of scenarios

investigated to an acceptable level.

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1.4. Methods

To be able to investigate the storage and reserve capacity needs of various mixes of

renewables, nuclear power and fossil fuels a model was built based on the

supply/generating profile for the year 2014, this data was used to estimate what

various factor increases in the generating capacity of each technology type would look

like if installed.

A more complete explanation of this process is contained in Section 3.

2. Literature Review

2.1. Pumped Storage

Figure 2 Wind Resources Across Europe

(Wikipedia, 2015)

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Pumped Hydro Storage (PHS) is a form of hydropower used primarily for load

balancing and energy storage that involves the pumping of water from a low lying

body of water to a higher one. Subsequently the water is run back down to the lower

reservoir to generate electricity. PHS does not on its own generate electricity and is in

fact a net consumer. It can better be thought of as a giant battery. Like any other

battery it has a round trip efficiency, normally in the region of 80% (Ter-Gazarian,

1994, p. 97)

It is currently still the only grid scale form of energy storage. Although battery,

compressed air and flywheel technology have come far in the last twenty years there

is nothing that approaches PHS capacity, storage levels and storage times.

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2.1.1. Turbine Types

A reaction turbine is the type most commonly used in pumped hydro as it has the

ability to run in reverse as a pump. In reaction turbines the mechanism that turns the

energy of the water into rotational energy (i.e. the runners) are fully submerged in the

water flow. The energy from the pressure differential resultant from this flow across

the runner vanes is used to turn the turbine.

Figure 3 Diagram of a Standard Francis Turbine

(NPTEL,2015)

Francis Turbine

The most commonly used reaction turbine in PHS is the Francis turbine (Figure 3),

used typically for heads of between 15-500m (Hermann-Josef Wagner, 2011,p.77).

The water comes through the spiral casing and is guided by the stay vanes efficiently

onto the runner section. The flow of the water causes a pressure differential across the

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runner vanes, which are specially shaped to utilise this and convert it into a shaft

torque.

Figure 4 Photograph of Large Francis Turbine

https://en.wikipedia.org/wiki/Francis_turbine

Note Figure 4 which shows the guide vanes of a turbine from the Chinese Three

Gorges Dam: The vanes, while being slightly curved at the top part of the vane to

produce the pressure differential, also tend to “bucket” at the bottom. Francis turbines

can also be called mixed flow turbines, as the water across the vanes flows both

axially and radially. (Hermann-Josef Wagner, 2011,p72). The water having left the

turbine then drops out of the draft tube which is designed to reduce the velocity of the

outrushing water to preserve pressure at the turbine exit. This prevents unwanted

backflow and allows the turbine to operate as close to the tail race as possible.

Kaplan Turbines

Kaplan turbines are the other type used in PHS, though these are much less commonly

seen in pumped storage systems. Kaplan turbines are better utilised for high flow

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rates, with low head heights of around the 25m range (Hermann-Josef Wagner,

2011,p87). This means that they are ideal for running between the very large volumes

with small altitude differences which are commonly seen in natural bodies of water or

in large river systems.

Figure 5 shows the basic schematic of a Kaplan turbine. They are axial flow and work

in a similar way to the Francis turbine in that the water flow is initially guided by

guide vanes to flow down through the turbine blades, in doing so lift forces which

turn the turbine are created.

Figure 5 Kaplan Turbine Diagram

(NPTEL, 2015)

2.1.2. Reservoirs

Reservoirs are perhaps the most critical aspect when designing a PHS as they dictate

the location of the system. Many factors must be taken into account when looking at

prospective sites such as environmental concerns, height difference between the

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reservoirs, and rock type upon which they will be built. With so many criteria it is no

wonder it can be a difficult task to find new PHS sites.

The upper and lower reservoirs can also sometimes be referred to as the forebay and

afterbay respectively. The approach with which the upper and lower reservoirs are

created can tend to differ in both location and in the type of landscape they utilize.

(Barnes & Levine, 2011, p63)

Upper Reservoir

The upper reservoir of a PHS can be obtained by using existing natural bodies of

water such as lochs or lakes, these can be further enhanced by damming them so as to

allow increased storage capacity. A river or stream can be damned upstream across

the valley floor, this is also suitable for creating a lower reservoir though great care

must be taken to allow for the release of floodwater, and in some cases fish (Ter-

Gazarian, 1994, p92).

An upper reservoir can also be created by damming a hanging valley that was

previously devoid of water. This allows a lot more potential sites as there is no need to

look for an existing body of water at suitable height. Cruachan in Scotland is a good

example of this.

An upper reservoir will have to deal with the natural inflows that will inevitably occur

from the surrounding landscape, and so a construction of a spillway is sometimes

necessitated to allow any additional water to leave the reservoir without any harm to

the system.

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Lower Reservoir

Lower reservoirs can be created in many of the same ways upper reservoirs can,

through damming of a valley or use of an existing body of water (either natural or

man-made). A very large river may be used for a lower reservoir as long as it can

cope with large volumes of water being depleted from and added to it without harm to

the local ecosystem. (Ter-Gazarian, 1994,p92)

There are two further possible lower reservoirs, both at the cutting edge of pumped

hydro systems. One would be an underground storage cavern. This would have the

advantage of greatly expanding the number of PHS sites available, though

engineering and cost limitations at the moment mean there are no real world

examples. The other is to use the sea as a lower resevoir. The Okinawa Yanbaru

Seawater facility in Japan is a 30MW station that pumps seawater 150m high for

storage and as of 2015 is the only one of its kind. Pumping seawater brings its own set

of challenges, such as enhanced corosion from saline water. Yanbaru has overcome

this through the use of stainless steel and fibre reinforced plastic. (Fujihara,1998)

Penstocks

The penstock is what carries the water down from the upper reservoir to the turbine

hall. In older systems it was commonly an outside pipe that went down the mountain,

though nowadays most systems have their penstocks kept inside the excavated

mountain. (Ter-Gazarian, 1994, p88)

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The best pumped hydro systems have a penstock length to head height ration as close

to 1:1 as possible. This would mean ideally the the entrance to the penstock would be

directly above the turbine itself (Barnes & Levine, 2011, p67). A more conventional

figure is 4:1 (Ter-Gazarian, 1994, p92).

Surge Tanks

Surge tanks branch off of the penstock and are located between the upper resevoir and

turbine. The purpose of a surge tank is to protect the water channels and machinery

from harmful changes in pressure, though it can also be used as a way of regulating

load. (Barnes & Levine, 2011, p69)

2.1.3. Pumped Storage in the UK

Worldwide capacity of PHS is in the region of 132GW with the UK contributing

2.8GW to this value. (EIA, 2012)

In the UK PHS was built for two reasons. One was as a way of storing nuclear power

overnight when demand was low. Nuclear power stations are mostly unable to throttle

their output, and so the pumped hydro stations would be able to use this excess power

to fill their upper reservoirs and then run the turbines during times of peak demand.

The other was as a quick response measure for grid stability as PHS can act as both a

large supplier and consumer of electricity in very short time frames. It is therefore

ideal for balancing out sudden surges or drops in demand.

There is a final use for PHS in the UK and that is as a ‘black start’ mechanism for the

grid. A black start is the process of returning a power station, part of the grid or even

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the entire grid to being fully functional (National Grid, p.8). As most power

generators require an external electricity feed before they themselves can begin to

generate power, the grid needs certain ‘black start’ capacity. This means that if a part

of the grid or the whole grid goes down, a supply of electricity can be provided to

allow the majority of other plant to begin generating electricity. PHS can provide this,

as to get it operational is as simple as opening up the pipeline gates (these gates are

normally powered by a backup diesel generator).

Current PHS Capacity

The UK currently has only four pumped hydro schemes, they are:

Table 1 Existing Pumped Hydro Storage in UK

power head volume energy stored

(GW) (m) (million m3) (GWh)

Ffestiniog 0.36 320–295 1.7 1.3

Dinorwig 1.8 542–494 6.7 9.1

Foyers 0.3 178–172 13.6 6.3

Cruachan 0.4 365–334 11.3 10

station

Source: (MacKay, 2008)

As the UK’s pumped storage was primarily built for overnight storage of nuclear

power and as a response to sudden surges/drops in demand, they are not designed to

store enough energy for more than around 20 hours at full output.

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Planned and Proposed new PHS

There are a number of new PHS schemes that have either been mooted or are in the

planning stage, they are:

Planned

1. Coire Glas: A 300-600MW SSE scheme that utilises Loch Lochy as its lower

tailpond and an artificial dam built 500m above. Storage capacity is expected

to be in the order of 30-40GWh. (SSE, 2012)

2. Balmacaan: Another 300-600MW SSE scheme with around 30-40GWh of

storage, this time utilising Loch Ness as the lower tailpond and an existing

Lochan that will be enlarged by a new dam or dams. (Lannen, 2012)

Proposed

3. Glyn Rhonwy Scheme: A proposed 600MW site situated in Snowdonia

national park that utilises old slate quarries with a head height of 300m

(Holmes, 2015)

4. Loch Sloy conversion: The current conventional Loch Sloy hydro site has

been proposed for conversion to PHS. If pumping capacity was added now

then the station could hold 20GWh, but with the dam heights raised by 40m it

is estimated that around 40GWh could be stored. (MacKay, 2008,p.193)

2.2. UK Renewable Intermittency and Variability

2.2.1. Wind Power Output

Wind power output is related to two main things: turbine size and wind speed. The

bigger the turbine, the more power it is able to produce when conditions are windy.

Also, generally the windier it is the more power will be produced, though most

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turbines will have a cut-out speed where the turbine will cease turning at very high

wind speeds to prevent damage to the equipment. Because power output is directly

affected by wind speed it means that given the generally stochastic nature of wind

then the power output of a wind turbine is itself stochastic.

Fortunately though, by having windfarms that are geographically spread throughout a

country or region, then fluctuations for the overall installed capacity will be smoothed

out to some extent.

Figure 6 Output of Single vs Distributed Windfarms

Source: (Boyle, 2007,p37)

Figure 6 illustrates this perfectly. It shows the output over a day for a single 1000MW

wind farm versus the output of 1000MW worth of geographically distributed farms.

By having a geographically spread out wind capacity the violent peaks and dips in

output can be mostly avoided. (Boyle, 2007,p36)

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Although a geographically diverse installed capacity will go some way to smooth out

the variability of output that is usually seen in wind, it cannot completely ameliorate

the effects that wind speed variability brings, as we will see for cases in the UK.

Wind in the UK

The UK has the best resource for both onshore and offshore wind in Europe (Figure7).

This still does not mean that wind resources in the UK are totally reliable or that we

are able to anticipate wind speeds over long time-frames with a very high degree of

accuracy.

Figure 7 Wind Resources Across Europe

(WASP, 2015)

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Winds in the UK tend to be stronger and less intermittent in the winter months, and

weaker and more susceptible to lulls and with lower wind speeds in summer. Also,

generally speaking wind speeds tend to be stronger during the day than during the

night (Sinden, 2005).

Figure 8 UK Wind Power Output in 2013

Figure 8 shows the total output of the UK’s entire fleet of wind turbines for the year

2013. Note that even in the winter there are massive variations in output over days.

Wind Forecasting

Just because the wind is intermittent and largely random it does not mean that we do

not have the ability to forecast the wind over very short timeframes. Figure 9 shows

the day ahead forecast of a Belgian offshore wind farm for the month of July.

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Figure 9 Forecast vs Actual Wind Power Output

Data Source: http://www.elia.be/en/grid-data/power-generation/wind-power

Note that although not completely accurate on a day to day basis, wind output was

broadly able to be accurately forecast, this means other generating capacity can be

brought on and offline in anticipation of falling and rising wind power.

Onshore and Offshore Capacity Factors

When discussing variability in the output of wind turbines it is also important to

highlight the difference in capacity factors between turbines sited onshore and those

offshore. The wind offshore tends to be stronger and so correspondingly power output

is greater. Also the wind is less variable and so offshore farms can supply a steadier

power output. (Markian M. W. Melnyk, 2009, p309)

In the UK, the average capacity factor for an onshore wind farm is around 26% while

offshore is 34% (DEAC, 2015) This means that on average, a turbine sited offshore

can expect to generate 1.3 times as much electricity as the same one sited onshore. In

terms of grid stability this means it is better to build offshore than onshore, but

economically it is much more expensive though prices will ineveitable fall as the

technology matures.

http://www.elia.be/en/grid-data/power-generation/wind-power

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2.2.2. Solar Power Output

The output of solar panels is affected by many different factors such as: latitude, angle

and pitch of panels, efficiency, panel temperature, shading or obstructions and dirt on

the panels. Some of these factors may be intrinsic to the location of the panels while

others are in some way controllable (cleaning the panels for example.) The main

factors in terms of grid stability we are concerned with is the local climatic conditions

i.e. direct and diffuse radiation at any one time, and the factors that affect them such

as length of day and cloud cover.

Unlike wind, solar power output can change very rapidly as when a panel is not

receiving sunlight its output falls to practically nothing in very short timeframes. This

means that when clouds move in they have an immediate and profound effect on solar

output. The effect can again, like wind, be partially offset by having an installed

capacity geographically spaced out.

Photo Voltaics benefit from being able to be located practically anywhere, from

rooftops to vacant fields, with little in the way of the environmental concerns that can

slow down, or stop, wind farms and thermal power plants being built. (IEA,

2003,p53)

Solar Intermittency

Just as with wind, the solar output of PV installations have high intermittency to deal

with.

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Figure 10 Annual Solar Output for Single 125W Panel

Data from merit Simulation

Figure 10 shows the output of a single south-facing 125W panel located in the south

of England over a typical year. Note that there are broad trends throughout the year,

with winter months having at times around one fifth the output in summer.

Variance/intermittency is especially seen on the hourly scale, Figure 11 shows the

output of a 250W panel in Cambridge over a day in August 2012

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Figure 11 Daily Output from 250W Panel

(CambridgeSolar, 2015)

Again, broad trends can be seen, with solar output gradually rising to a peak at mid-

day before falling again until sunset. However within the broad trend, there are

numerous dips as cloud cover comes in and goes out.

The factors which affect solar output hour to hour are climatic and geographic in

origin. Time of day and season are the largest reasons for the biggest differences

locally, while more generally the location of the panels has an effect. For the UK

generally speaking the farther south the greater solar output the panels will have

(Figure 12)

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Figure 12 Solar Resource Across the UK

Image Courtesy of the Met Office

When contrasting Figures 8 and 10 we can see that solar power is less intermittent

than wind and that trends throughout the year are generally predictable. The major

problem with the intermittency of solar is that when the sun goes down there is no

power. Also in the dark, cloudy winter months, effectively again solar output is

approaching zero - this unfortunately coincides with when demand is at its peak, this

will have consequences for grid stability.

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Solar Power in the UK

As of August 2014, the UK has an installed solar capacity of 5GW with more than

half of it located in the south of the UK where solar radiation is highest, and therefore

variability is lowest (Figure 13).

Figure 13 UK Regional Population of Solar Panels

Image courtesy of Solarbuzz

Figure 14 shows where the installed capacity is located, with a third being on

residential homes, and 45% located as ground mounted systems. The remainder is

located on commercial rooftops. The geographically diverse nature of the installed

capacity of Solar PV in the UK will be beneficial in terms of a smooth total power

output as local climatic conditions will be offset by areas with different conditions

owing to the distances involved.

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Figure 14 Application Split of Solar PV

Image courtesy of Solarbuzz

2.2.3. Hydro Output

Hydro power was the first large scale power generator, and even today it is the largest

source for renewable energy in the world. As opposed to pumped hydro storage, when

we talk of hydropower we mean usually run of the river hydro or a dammed river with

no ability to pump the water back up for storage. In terms of intermittency and

variability, hydropower is, except for perhaps biomass, the most consistent of the

renewables. It has a worldwide load factor of around 44% (Elliot, 2013, ch2 p1),

although this can vary from site to site. The Three Gorges Dam in China has a load

factor of 50%, while the Hoover Dam has one of 23%.

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Hydropower output is, like wind and solar, related directly to climatic conditions,

more specifically rainfall. Rainfall varies a lot less than wind, and seasonal changes

are broadly forecastable although with climate change more severe weather

fluctuations may lead to increased difficulty when trying to anticipate futre

hydropower output. Also hydro sites with dams have the ability to control their output

by storing water behind the dam, so power generation is more even.

Hydro in the UK

Hydro power in the UK is very small, as it only makes up around 1.5GW of installed

capacity, with an average load factor in the region of 38%. It can fall as low as 22% in

years with very low rainfall. (Elliot, 2013,Ch2 p2)

Figure 15 UK 2014 Hydropower Output

Source:Gridwatch

Figure 15 shows the output of the UK’s conventional hydro plants for the year 2014.

There are clear stable trends to the output throughout the year, with a steady value of

around 1GW for the winter months, and a slowly falling output from spring to

summer that bottoms out around 0.2GW before beginning to rise again in autumn.

34

Although there is some variability, in power production terms this is a fairly stable

output.

2.2.4. Nuclear Power Variability

Nuclear power, though not a renewable source, is relatively carbon free and so a

popular option amongst some policy-makers and scientists in that it goes alongside

renewables as the best way to a carbon free power system. In terms of variability,

nuclear power’s problem is not that it is very variable, but that it is in fact not

variable. To a large extent nuclear power has no ability to load follow; the output of

reactors cannot modulate to suit demand as a nuclear reactor runs most efficiently and

cheaply when it is run continuously and as close to its rated capacity as possible.

This means that nuclear is suitable to meet base load demands in the grid, but is

unsuited to respond to sudden surges and dips seen throughout the day, or even to deal

with the lower demand requirements at night. One way previously discussed to get

around this is to use nuclear in conjunction with PHS so that nuclear energy produced

during the night when there is no demand can be stored for times of peak demand.

France gets around 90% of its electricity from nuclear power. It handles its load

following by use of “grey" control rods which slow down reactions within the reactor

without the need to clean the water after Boron control rods are used, though this does

have a corresponding impact on overall efficiency and therefore cost (World Nuclear

Association, 2015).

35

France also can deal with gluts of nuclear power by exporting to other countries such

as Germany, Belgium, the UK, Italy, Switzerland, and Spain.

Nuclear Power in the UK

The UK has just under 9GW of installed nuclear capacity spread out over 16 plants.

They contribute around 19% to the total electricity generated in the UK over a year

(DUKES, 2012). For the reasons previously discussed, nuclear power plants in the

UK are run continuously and as close to maximum capacity as possible. Figure 16

shows the output of the UK’s fleet of nuclear power plants over 2014. For most of the

year, the nuclear fleet will put out around 8GW of power, though there are variations

in output as plants come offline for scheduled maintenance or because of

malfunctions. This means that even though nuclear is generally seen as a reliable

steady source of power, it still does need backup generating capacity to deal with

these outages (World Nuclear Association, 2015). In fact, according to the UK’s

Department of Energy and Climate Change, the average nuclear load factor between

2007 and 2011 was 60% (DECC, 2012)

36

Figure 16 UK 2014 Nuclear Power Output

Source:Gridwatch

2.3. Grid Stability with Renewables

As we have seen in the previous sections, the carbon free generating capacity brings

problems of variability and lack of ability to modulate output to suit demand in any

great amount. As we move towards a carbon free generating mix in the UK, this

variability and inflexibility will put an ever increasing strain on the national grid.

2.3.1. An overview of the Grid

In an electric grid, power is generated, transmitted, distributed and then consumed.

Conventionally AC power is generated at a large station, where the voltage is kicked

up to very high voltages for transmission to reduce electrical losses. The power is

transmitted along transmission lines to substations, where the voltage is again kicked

back down for distribution and then consumption.

37

For all electrical grids, the overall power output of the system at any one time must

closely match the demand at that time. If there is not enough power being generated to

meet demand, this leads to a “voltage dip” which means the frequency of the power

being supplied dips for a short period of time. This can lead to what is known as a

brownout - lights can dim and motors can slow down. This can be damaging for

modern electronics that rely on a high quality power source to function correctly. On

the other side, where generation exceeds demand, this can lead to a power surge,

where frequency of the electricity goes above acceptable levels. This can lead to total

blackouts if the surge is great enough to trip safety features, or even damage

transmission equipment. If it is a minor surge, it can lead to an increase in voltage in

all devices across the network, leading them to burnout or trip their fuse breakers.

Figure 17 Typical Grid Layout

Source: http://www.democratandchronicle.com/

Figure 17 shows the typical configuration of a national grid with the generating,

transmission and distribution equipment shown.

38

Fuel Use in the UK Grid Today

In the UK today, we have a mix of conventional fossil fuels making up the bulk of our

generating capacity, but with an ever growing amount of renewables being added into

the mix.

Figure 18 Electricity by Fuel Source 2014

Source: (Department of Energy and Climate Change, 2015)

Figure 18 shows the makeup of the UK’s electrical output by fuel type. Nuclear and

coal will provide base load, while the renewables will contribute whenever they are

available. Gas will modulate to keep the supply-demand balance even.

39

Figure 19 Type of Electricity Generated 1980-2014

(Department of Energy and Climate Change, 2015)

Figure 19 shows the trend in the UK of decreasing oil and coal use, with increasing

amounts of gas and renewables in the mix.

Transmission Infrastructure in the UK today

Figure 20 UK Electricity Transmission Network

Source: National Grid

40

The UK, when compared to most other Western countries has an aging grid network

in need of modernisation; its ability to transmit large amounts of power is severely

limited by the bottlenecks in the transmission networks. (IEA, 2011) Most of the

UK’s infrastructure was built in the ‘50s and ‘60s and so was not designed for a low

carbon generation mix. OFGEM have suggested that the cost of improvement could

be as much as £32 billion. (OFGEM, 2010) There are currently no 400kV lines that

run from the places where the wind, wave and tidal resources are greatest (the north of

Scotland) to the population centres where demand is greatest (the south of England).

Figure 20 shows the status of the national grid as it stands today. Note the dearth of

400kV lines in Scotland, where wind and hydro resources will be greatest.

Figure 21 Comparison of Grid Strength in Various Countries

Source: (IEA, 2011,p78)

The diagram above shows an appraisal by the International Energy Association of

various countries’ grids’ ability to handle variable renewable energies (VREs). The

41

UK lags behind its northern European neighbours. If the UK wishes to greatly

increase the amount of energy it receives from variable renewable sources, it will

have to spend money on expanding and reinforcing its grid infrastructure. Luckily it

has already began this process, with the 400kV Beauly-Denny line that runs from the

north of Scotland to the central belt, where it can connect to other 400kv lines that

lead to the population centres in England.

UK Demand Characteristics

The UK has a fairly typical demand profile, seasonally peaking in winter when

demand for electric heating is greatest, and on a day to day basis having a small peak

in the morning when people wake up and begin their day, followed by another peak in

the evening when most people arrive home after work. For the most part this is

broadly forecastable, though something as innocuous as a TV programme being more

popular than anticipated can lead to sudden surges.

42

UK Typical Annual Profile

Figure 22 Typical UK Annual Demand Profile

Source:Gridwatch

The UK varies annually between a peak during winter evenings of around 53GW and

a minimum during the summer nights of around 20GW.

43

Figure 23 Typical UK Daily Demand Profile

Source: Gridwatch

Figure 23 shows the typical fluctuations seen in the national grid over a day. This was

for a day in April, so the lowest and highest demands are between those you would

expect to see in winter and summer.

Grid Resilience Measures

There are a number of measures the national grid has in place to make sure the

frequency of a grid is kept at a steady 50Hz (National grid deems a variance of

±0.5Hz to be within acceptable limits)

44

Pumped Storage

As discussed previously, PHS can be used to respond to sudden surges and drops in

demand. It has an almost instantaneous response time, and also has the ability to store

excess power that would otherwise have to be ‘spilled’.

Spinning Reserve

Spinning reserve is the process of having plant (usually CCGT) putting in power to

the grid but at a certain fraction of its total possible output. This means that if there

are sudden surges in demand, the spinning reserve can come up to full power in a

short period of time to meet the demand. If the plant was simply switched off, it could

take up to an hour before the plant was ready to input into the grid. Note that because

the plants are only operating at part load, there are negative consequences in terms of

efficiency leading to increased cost and higher carbon emissions per MWh produced

than would be the case for plant operating at close to maximum.

Fast Start: Generation Units

This involves the start-up from cold of certain generating capacity, usually diesel

generators owing to the relatively short time they need to start up before they are

ready to input into the grid.

NG Frequency Service

This is a system whereby in exceptional cases of demand, the national grid can ask

certain large scale consumers of electricity, who have agreed to the service in

exchange for financial reimbursement, to suddenly switch of their power supply. An

example of this could be a steel mill or manufacturing plant.

45

Reserve Capacity

Reserve Capacity involves open cycle gas turbines (OCGT’s) and oil generators that

will sit unused for most of the year and only be switched on in times of extreme

demand, usually only seen in especially cold winters.

The below graph in figure 24 shows the operation of OCGTS’s during 2014, you can

note that they are used sparingly throughout the year and mostly for short bursts in the

winter.

Fig 2.3.8

2.3.2. The Challenge of Variable Renewables

In previous sections we have discussed how and why renewable sources are variable;

we have also seen how the national grid must constantly balance the supply and

demand sides of its network. As renewables play an ever increasing role in the way

we generate our electricity this balancing act will become more and more difficult.

Figure 24 OCGT use in 2014

46

Fluctuating Supplies

As previously seen, generally sources of renewable power fluctuate in their output

timescales as short as seconds or as long as months. This means that currently for the

supply and demand sides of our grid to match, we must have enough flexible capacity

that can modulate its output quickly to maintain this balance. Currently with the very

small amounts of renewables in our generation mix, this has proved simple enough as

the fluctuations on the demand side will vary much more than on supply side. (IEA,

2011, p31)

Going forward into an age of large penetrations of renewables in our generating

capacity will mean the need for either more forms of energy storage, increased

flexible and reserve capacity on the generating side, or increased demand

management in the grid where we tailor our demand to suit supply rather than the

opposite case we currently experience.

Power When You Do Not Need It

As you have no control over when renewables can produce energy (with the exception

of some forms of hydro), economically speaking it is best to be able to make use of all

the resource available. As more and more wind is installed in the UK there will come

a time where large amounts of wind are generated at a time when no-one is

demanding it. When this happens companies have no option but to “spill” the wind.

This effectively means that some of the turbines are turned off so that they do not

overload the grid. This can be seen as wasted energy, and from an economic view

adds to the cost of renewable systems as they are not paid for electricity they could be

generating.

47

Too Little Power When You Do Need It

The opposite of this is that, no matter how geographically diverse the renewables are,

there will be times when output from them is effectively zero. For solar this happens

in the long dark, cloudy, winter months when demand is highest. In the UK there will

be days with no wind and even sometimes large anti-cyclones, where for anything up

to a month no wind will blow over the entire country. In a system where renewables

make upwards of a 40% contribution to the supply side of the grid, this could lead to

major blackouts and all the social and economic turmoil they bring.

Unpredictability

Obviously there is already some amount of inherent unpredictability in our national

grid. Power plants can malfunction and come offline at any time, damage to

transmission equipment can occur, and some surges in demand can be unpredictable.

However, with the inclusion of renewables in the mix, it makes it much harder to

operate the grid. Instead of having to deal with sudden surges and falls in demand and

the occasional equipment malfunction, you have to deal with a constantly changing

supply. We have seen how renewable output can be forecast, but even very small

changes in output must be taken into consideration to avoid dips and rises in

frequency.

2.4. Storage with Large Renewable Penetrations

Storage can function in two useful ways when dealing with the problems renewables

can bring to the grid. The first is in load balancing, as we have seen with PHS storage

which can act as both a load and a supply. This is very good for grid stability as it is

48

no longer reliant on backup capacity that takes time to come on and off. With PHS it

is a matter of seconds before it can draw from the grid or supply to it. Storage allows

the peaks and troughs seen in demand to be smoothed out; this creates a more flexible

responsive grid.

The second role is in storing energy. The less wind you have to spill or solar panels

you have to disconnect, the more efficient the system is. Storage increases the

efficiency of renewables by taking power from the supply side at times of increased

output and storing it for use at times of great need. This means no wind or solar

resource is wasted, while also reducing the need for additional capacity of renewables

at times of peak demand.

3. Modelling of Grid

To investigate the amount of PHS that would be required for various penetrations of

renewables and nuclear in the generation mix, a model was built. The model would

allow a user to input various factorial increases in the current installed capacity of

various generator types - 3x current wind capacity, 2x nuclear etc. The result would

then show you how much storage would be required such that supply always met

demand. In the case where storage runs dry, the model would then show how much

backup CCGT would be required to make up for the renewable and storage shortfall.

3.1. Demand and Renewable Data Source

The source of the data most of the supply and demand was modelled on was from the

balancing mechanism reports the national grid releases (http://www.bmreports.com/)

49

they provide real time data from the national grid in five minute increments; the data

is archived and was available for the years 2010-2015. The actual data the model

utilised was downloaded from the Gridwatch website, a site that takes the BM reports

from national grid and displays live the demand and supply makeup of the national

grid. It also collates all the data from previous years into excel form.

The datasets included information on

demand

grid frequency

coal generation

nuclear generation

CCGT generation

wind generation

pumped hydro generation

conventional hydro generation

biomass generation

oil generation

OCGT generation

French imports/exports

Dutch imports/exports

Irish imports/exports

To simulate different capacity increases in nuclear and the various renewables, the

output for each type of technology over the year was multiplied by a chosen factor.

For instance, if on a specific date and time the output from wind turbines was 3GW

and the model wanted to see what a capacity increase of 3x would look like, it would

50

simply multiply the figure by three. There are assumptions and consequently

drawbacks to this method that will be discussed in the weaknesses section.

The pumped hydro was looked at by allowing a pre-selected maximum storage level

in GWh. Any excess power above demand was then put into the pumped hydro

column until it reached this maximum figure allowed, and when the maximum is

reached the model then dumps the remaining excess energy into the curtailed

renewables column. Conversely, when the PHS column reached zero and supply from

generating capacity and storage failed to meet demand the model would take this

energy deficit and use it as the amount of energy that the gas turbines would have to

supply. A round trip efficiency of 80% for the PHS was implemented as this is a

common efficiency, to avoid putting additional energy into the system that wouldn’t

have been necessarily otherwise generated it was assumed the energy store would

start the year empty.

This overall allowed the model to simulate what effect larger capacities in renewables

and pumped hydro storage size would have on fossil fuel use.

3.2. Solar Modelling

Solar power is one of the fastest growing renewable technologies in the world. The

price of solar has come down by the most amount in the shortest time of any

renewable in history. In five years alone, the UK solar capacity went from effectively

zero to 5GW. As solar performs best in summer, many see it as a complementary

technology in the UK to wind which maximises output in the winter months.

51

Unfortunately the BM reports do not provide data on solar generation, but merely

showed up any solar input into the grid as a fall in demand. Nevertheless it was felt

that to omit solar power from the model would hamper its ability to forecast the future

generation scenarios in the UK.

To simulate solar input into the grid, a series of Merit simulations were run using

various locations in the UK. The simulation was run for a 2kW south facing system in

each region of the UK. The results were then factored up so that they represented

5,000,000 homes or 10GW of installed capacity. The results from the different regions

were then weighed to the percentage of panels installed that each region currently

possessed and subsequently combined. This would best reflect the geographical

distribution of solar panels any increase in capacity would entail. The total output for

all regions was then inputted into the model. The end result is that 1x factor input into

the model will equate to 10GW of installed capacity installed across the UK but with

most panels centred on the south.

3.3. The 2014 Base Case

Except for the solar data which for reasons previously mentioned had to be simulated,

and the CCGT data which works purely in response to the balance between supply,

demand and storage, all other data was based on the actual operating conditions of the

grid during the year 2014.

52

3.3.1. Wind

By the end of 2014 the installed capacity of wind turbines in the UK was 12.44GW.

This was a 13% rise in installed capacity on the previous year and was composed of

roughly 60% onshore and 40% offshore. (EWEA, n.d.)

The actual wind resource in the UK suffered from two long lasting high pressure

systems. One at the end of August which lasted well into September, and as such, as

Figure 25 shows, wind power at this time struggled to get above the 2GW mark. This

happened again in December, where for the first week in December again wind power

was substantially lower than what could be normally expected at that time of year

(MetOffice, 2014). Although all simulations will show this massive drop in wind

power, it is not seen as a weakness in the model as high pressure systems are not

uncommon in winter in the UK and by having two in in relatively quick succession

allows the model in terms of wind power to be based on a ‘worst case’ scenario.

Figure 25 Wind Power Output Winter 2014

53

Still overall electricity supplied from wind was up 20% on the previous year which

allowing even for the increase in capacity was still better than 2013. (BusinessGreen,

2015)

3.3.2. Nuclear

During 2014, the UK had an installed capacity of 9.3GW of nuclear power, this

consisted of 16 plants at 8 separate sites. Figure 16 showed the output of nuclear

power during 2014, there is a steady drop in nuclear power seen between August and

October which is explained by the fact that during this time, four reactors at

Heysham and Hartlepool had to be brought offline for eight weeks due to a fault being

found in a boiler unit (BBC, 2014). This again is not an uncommon thing to happen,

and in fact for 2013 similar drops in nuclear output were recorded at different points

during that year. Therefore it was felt these drops added a robustness to any model the

data was built upon, as any new nuclear capacity will be subject to a similar amount

of downtime due to fault or maintenance and so to remove or partially offset the drops

in output would in fact detract from and scenario created by the model

3.3.3. Coal

Coal accounted for 101TWh of energy production in the UK, around 30% of the total

electricity generated that year. (DUKES, 2015,CH5) Installed capacity was around the

20GW mark, though this decreased over the year as some stations were

decomissioned or converted to biomass. In terms of variability coal had a fairly steady

output of 16GW in the winter months and getting as low as 5GW in summer, this is

due to the fact that although coal power stations cannot load follow in the short term,

they can be brought offline for seasonal changes in demand.

54

3.3.4. Conventional Hydro

The UK in 2014 had 1.65GW of installed hydropower. For the start of the year, hydro

power put out a fairly consistent 1GW, beginning to fall in the spring months to a

summer output of 0.4GW before rising again to 1GW for the winter months. In 2014,

hydro generated a record 6TWh of electricty, rising 25% on the previous year

(DUKES, 2015, ch6). This was an especially strong year for UK hydro power as the

year was exceptionally wet and would not be seen as typical year for the UK

(Guardian, 2014).

Given the small amount of installed capacity hydro power represents to the total

generating capacity of the UK, it was deemed this would not harm the model.

3.3.5. Biomass

Electricity generated from biomass rose from a 2013 value of 4,176 GWh, to 13,105

GWh. This represented a 300% increase, mostly due to a unit at DRAX coal fired

power station being converted to biomass. Output was a fairly consistent 1.1GW,

although there were four instances where output dropped to 0.8GW when stations

came offline for maintenance. (DUKES, 2015,ch6)

3.3.6. CCGT

As of 2014, the UK has around 32GW of CCGT capacity (DUKES, 2015, Ch 5.6),

although the maximium output of CCGT’s was around 24GW. In total for 2014,

CCGT’s supplied around 86TWh of electricity to the UK grid, or around a quarter of

total power generated.

55

3.3.7. Other Supply Options

Some supply options in 2014 were left out of the model. All exports were left out as it

would be difficult to anticipate another country’s ability to supply or receive power.

OCGT and oil burners were left out as they are only used in emergencies, and so with

different capacities installed and different storage levels, their 2014 operating times

would bear no resemblance to any future scenarios.

3.3.8. Demand in 2014

Consumption of electricity was down 5.6% on 2013 with a total generation of

339TWh. This fall is fairly consistent with current trends. This creates a problem with

the model, as it bases future demand on the 2014 case. It is possible to change demand

by factors as was done with generating capacity, but as it is difficult to extrapolate

whether fall in demand would be uniform over a whole year or affect certain seasons

more than others it was decided to keep demand identical to 2014.

3.4. Weaknesses

In the solar data there are obvious flaws in the way it was created. It would not take

into account the ‘smoothing’ effect which having geographically diverse panels in

each region would have on output. Also, different test reference years had to be used

for each location as there was no single test reference year for the UK as a whole. It

was felt though as ‘test reference years’ were picked to be what was seen as the

typical climate of each location in any given year, then combining them would not

detract from the strength of the model when compared to leaving solar out.

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The wind data will not be totally representative of what the various increases in

capacity investigated would actually look like. In 2014, the UK had an installed wind

capacity of 12,440MW. The majority of this is onshore wind, and as of 2014 around

5,000MW of the total is offshore. As the model simulates increased capacity based on

2014 data, then it will model for a continued 60% onshore 40% offshore split. From

recent government actions and planned offshore expansion, it looks likely this ratio

will swap. Unfortunately the model will not be able to simulate for the increased

capacity factor that offshore farms will have.

Figure 26 Geographical Distribution of Existing and planned UK Wind Farms

Source: Map Template by Derek Eder, Data by RenewableUK

Also with any increase in onshore wind capacity, the model will replicate the

geographical distribution of current farms. This may not be a large weakness because

57

as Figure 26 shows, all currently planned and under construction onshore wind farms

follow this pattern.

3.5. Scenarios Investigated

The model allows input of any scenario of installed renewable, nuclear and storage

capacity and will show the resultant amount of curtailed renewables and the amount

of backup CCGT needed in both terms of maximum capacity and energy out over the

year in GWh.

A set of scenarios have been devised to show the impact of carbon free technology on

the grid, and also the role of storage in helping to reduce curtailment of renewables

and reduce the need for CCGT to provide load following in the grid.

Scenarios 1-4

Figure 27 Scenarios 1-4 Factor increases in Capacity

Scenario 1 Wind Nuclear Solar Hydro Biomass Coal

Factor

increase to

2014's

capacity:

3 1 0 1 1 0


Factor

increase to

2014's

capacity:

6 1 0 1 1 0


Factor

increase to

2014's

capacity:

9 1 0 1 1 0


Factor

increase to

2014's

capacity:

12 1 0 1 1 0

58

The first four set of scenarios were intended to investigate the value of storage for

various capacities of wind. Scenario 1 had 3x the 2014 installed capacity, with each

subsequent scenario increasing the installed wind capacity by 3x the 2014 value. At

12x the 2014 capacity it would be the equivalent of almost 145GW of wind. This

would entail installing turbines in around a third of shallow waters and a third of 25m-

50m waters around the UK. (MacKay, 2008, p60-62) This increase is unlikely to

happen for obvious reasons, but it is helpful to analyse extreme examples for a more

complete picture of storage’s role in balancing renewables. The grid was kept

identical to 2014, except that coal production was put to zero to allow a better look at

how renewables could offset fossil fuel use.

Scenarios 5-8

Figure 28 Scenarios 5-8 Factor increases in Capacity


Factor

increase to

2014's

capacity:

1 0 3 1 1 0


Factor

increase to

2014's

capacity:

1 0 6 1 1 0


Factor

increase to

2014's

capacity:

1 0 9 1 1 0


Factor

increase to

2014's

capacity:

1 0 12 1 1 0

These scenarios were identical to the first four, but it was solar capacity instead of

wind that was increased. It should be noted that as the solar data is based on MERIT

simulations and not on actual solar data from 2014 so 1x entered into the scenario

59

simulator does not equate to 1x the 2014 installed capacity of solar. In fact between

Q3 2013 and Q3 2014, solar installed capacity increased by around 67% (SolarBuzz,

2014). This rapid increase would have made it difficult to model had the data been

available from the national grid as information on what the capacity was at any time

of the year was unable to be obtained. Instead the 1x factor equates to 10GW of

installed solar spread across the UK, but centred on the areas where most solar is

currently installed. This is the equivalent of around 2.8 million homes having the UK

average sized system of 3.5kW.

Scenario 9

Figure 29 Scenario 9 Factor increases in Capacity


Factor

increase to

2014's

capacity:

6 1 6 1 1 0

Scenario 9 was designed to see what role storage would play if the grid had a mix of

both wind and solar, given the rapid fall in the price of solar seen in the last few years

and the commitment to continued offshore wind indicated by the current government.

This scenario was deemed to be more likely to represent what the grid will look like

in the future when compared to the previous scenarios which concentrated on the

increase in capacity of one renewable only.

Scenario 10

60



Factor

increase to

2014's

capacity:

4 2.17 3 1 1 0

This scenario was designed to replicate the fact that the UK plans create around

19GW of new nuclear to replace old plants. As Sizewell-b is not expected to be

decommissioned until 2035, this would equate to an installed capacity 2.17 times that

of 2014.

Scenario 10 had reasonably realistic increases to wind and solar. The wind value

represents about what would be installed if all rounds of offshore wind are completed,

and an additional 4GW of onshore put in place as well. The solar value was the

equivalent of eight and a half million homes having the UK average solar setup of

3.5kW installed.

Scenario11



Factor

increase to

2014's

capacity:

3.3 1 3 1 3 0.8

This scenario was designed to replicate what the grid could look like in the next 10

years. The 3.3x wind capacity equates to the installed wind capacity once all rounds

of offshore have been constructed, and assuming approximately one third of the

planned 2500 onshore turbines in the planning stage get built. Coal use has decreased

by one fifth which equates roughly to 4GW. This 4GW has been replaced with

61

biomass as 3 times the current capacity will be just over 4GW. The solar as in

scenario 9 equates to 8.5 million homes. This may seem a lot, but the price of solar

has been steadily dropping and installation of panels has been rapidly increasing.

Scenario12



Factor

increase to

2014's

capacity:

10 0 5.25 2.722 5 0

The purpose of Scenario 12 was to look at what a grid completely free of coal and

nuclear base load would look like, and what role storage would play in it. All

renewables have been increased by the amount that would be theoretically possible,

but certainly not likely to be installed. Hydro has increased to 2.72x its current

capacity as some studies say there is an additional 2.841GW of unexploited

hydropower in the UK, mostly in Scotland (DECC, 2008).

Biomass has increased fivefold to around 6GW of installed capacity, though it

remains to be seen whether the amount of solid or gaseous fuel required for such a

system could be obtained sustainably.

Scenarios 13-15

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Figure 33 Scenarios 13-15 Factor Increases in Capacity

Scenario 13Wind Nuclear Solar Hydro Biomass Coal

Factor increase to

2014's capacity:1 4.7 0 1 0 0


Factor increase to

2014's capacity:5.2 2.17 2 2.722 5 0


Factor increase to

2014's capacity:9.6 0 6.5 2.722 5 0

These scenarios were to show how much storage would be required to entirely

manage without CCGT’s load following abilities. They were not attempts to replicate

any foreseeable scenario in the future, but to show how much storage is required to

replace gas turbines’ load following capabilities. The various scenarios involved:

Scenario 13: Storage requirements if all generating capacity except already

existing wind and hydro were replaced by nuclear.

Scenario 14: Storage requirements if all fossil fuels were replaced by a

nuclear/renewable mix.

Scenario 15: Storage requirements if all generating capacity was renewable

and there was no need for backup CCGT.

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4. Results

4.1. Scenarios 1-4

Table 2 Scenario 1 Results


Factor increase to

2014's capacity:3 1 0 1 1 0

37GW

Storage Amount GWh0 200 400 600 800 1000 1200 1400

CCGT Max Capacity

Needed GW41 41 41 41 41 41 41 41

CCGT Output over year

GWh166287 166287 166287 166287 166287 166287 166287 166287

Percentatage decrease in

CCGT use from no

storage

0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%

CCGT Operation

Reduction from 2014-92.71% -92.71% -92.71% -92.71% -92.71% -92.71% -92.71% -92.71%

Percentage Renewables

Spilled0% 0% 0% 0% 0% 0% 0% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

62.19 62.19 62.19 62.19 62.19 62.19 62.19 62.19

% of energy supply from

renewables25% 25% 25% 25% 25% 25% 25% 25%

For scenario 1, the level of storage made no difference to the amount of spilled

renewables or consequently reliance on CCGT. The wind resource was not enough on

any day to do more than partially offset CCGT use, so this meant there was no excess

energy to store. The 3x capacity is just less than what can be expected once all rounds

of planned offshore wind farms have been completed. Without coal, reliance on

CCGT has almost doubled, but due to the extra wind and gas turbines’ lower carbon

emissions, the grid has actually cut around 60 million tonnes of carbon dioxide. As

64

previously stated, there is only around 25GW of CCGT available currently, so this

scenario is unlikely to happen unless the price of gas continues to fall. With this level

of capacity only one quarter of the energy supply is from renewables, so storage will

not be much of an issue. This is why it is useful to simulate higher penetrations of

wind.

The levels of wind penetraion in Scenarios 2-4 did have enough renewable energy to

merit storage. Figure 34 shows the extent to which storage affected the amount of

work CCGT’s had to perform to balance the grid.

Note that the decrease in CCGT use is compared to if there was no storage in the

system not the decrease in CCGT use compared to 2014’s CCGT output.

Note that the more wind capacity you have in the system, the more effective storage is

at reducing reliance on gas turbines, and also the more useful storage remains at larger

levels of energy storage. For all scenarios, eventually the value of storage becomes

Figure 34 Decrease in CCGT Use When Compared to having No Storage

65

one of diminishing returns. The reason for this is that a small amount of storage will

be utilised often, as there will be many times over a year when windpower output

necessitates storage of 100GWh for brief periods of time. However, incidences where

the wind power necessitates 1200GWh of storage may only happen two or three times

in a year.

Figure 35 250GWh PHS System Use Throughout Year

Figure 35 shows the energy levels in a PHS system with wind power feeding into it.

The green line at 75Gwh has many times throughout the year where the store goes

above it, meaning the store is getting depleted more and so is able to store energy

multiple times. The red line which is at the 200GWh level is used four times in a year,

so this is an extra 125GWh of storage to store maybe three lots of 50GWh.

From the simulations it also shows that the size of the energy store makes no

difference to the maximum power output of the backup CCGT. This is because the

times when the CCGT needs to put out maximium power coincides with the times

when the energy store is depeleted i.e. when there are no renewables feeding into the

system. For the scenarios run, this occurred in December, when the previously

66

mentioned high pressure system occurred. At that time, wind energy input was

effectively zero for a week, while demand was high due to the fact it was in the cold

winter months.

Figure 36 Scenario 3 Energy Store Level 14-17 November

The graph above shows the storage levels for the scenario 3 case with a PHS amount

of 1400GWh. In just three days the store went from full to empty, and as the lull in

wind lasted weeks this meant that the CCGT had to do the job of 112GW of installed

wind capacity.

In terms of carbon emissions, Scenario 2 led to an increase in use of CCGT of 29%

though with an additional 200GWh of storage this would fall to 22%. The maximum

output of the CCGT turbines was 40GW wich was practically identical to scenario 1.

Even though it had double the wind capacity of scenario 1, it made little difference to

the need for size of backup capacity, as when the wind resource is not there it makes

67

little difference to the number of turbines you have. In terms of of spilled renewables,

just 200GWh of storage prevented 5% of all renewable energy being curtailed.

Scenarios 3 and 4 had what could be deemed unrealistic levels of installed wind

capacity, but they made the biggest difference to carbon emissions. Scenario 3 used

backup CCGT 8% less than was used in 2014, despite having no coal to provide

baseload. This jumped to 25% with the addition of 400GWh of storage preventing 7%

of renewables being spilled.

Scenario 4’s storage levels affected the use of CCGT backup the most. This was due

to the effect the system had on the ability to utilize the high storage levels more

throughout the year. With no storage, CCGT use was reduced 30%, but if the full

1400GWh of storage was available this would increase to 65%. 1400GWh would be

equivalent of 35 Coire Glas’s, (SSE’s planned new PHS system in Scotland). This

amount of PHS would be unlikely to be built in the UK due to environmental and

financial issues.

4.2. Scenarios 5-8

For scenarios 5 and 6, due to the fact coal was missing from the generation mix, the

need for storage was minimal much like the case was in scenario 1. The renewable

output was only enough to partially offset CCGT use, and so there was no excess

energy to store. It is still interesting to note however that the increased solar capacity

in scenarios 5 and 6 led to a cut in carbon emissions of 57 MtCO2e and 67 MtCO2e

respectively. This will partially be also due to the fact that coal use has been replaced

once again with gas turbines, which have lower carbon emissions.

68

Figure 37 shows, that the diminishing value of storage at larger levels is much more

extreme in the case of solar power than with that of wind. Again, like wind, the more

installed capacity of the variable source, the more valuable storage is, but even for

systems with enormous capacities of solar power anything past 200GWh of storage is

unnecessary as it makes little difference to the decrease in backup CCGT use. This

may be because unlike wind turbines, which can go through days of mostly

continuous operation, solar power stops at sunset. This means the store will

continuously fill and deplete every day, so that a higher storage amount is not needed.

Figure 37 Decrease in CCGT Use Compared to Having No Storage

69

Figure 38 Solar Energy vs Pumped Storage Energy 18th

March Scenario 8

Figure 38 shows the amount of energy in storage compared to the energy being

supplied form solar panels for a day in March for scenario 8. This shows why even

with large capacities of solar inputting into the grid, large amounts of storage is not

required. The energy store begins filling about 10am, as demand from the early

morning lowers and the power from the sun increases. The store reaches its peak

amount around 5pm, just as the sun begins to wane and evening demand surges.

These daily demand patterns coinciding with the waxing and waning of solar

resources means that the energy store is regularly being filled and depleted. This

means that volumes of storage which would allow storage over days and weeks are

not neccesary.

Again as with wind power, the storage size in a grid with only large amounts of solar

has no bearing on the size of backup capacity in terms of power , as in winter the solar

resource is next to zero and CCGT must make up the shortfall.

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4.3. Scenario 9



Factor increase to

2014's capacity:6 1 6 1 1 0


CCGT Max Capacity

Needed GW40 40 40 40 40 40 40 40


GWh72203 55753 51816 49863 48647 47473 46645 46244


CCGT use from no

storage

0.00% 22.78% 28.24% 30.94% 32.63% 34.25% 35.40% 35.95%

CCGT Operation

Reduction from 201416.33% 35.39% 39.95% 42.21% 43.62% 44.99% 45.94% 46.41%


Spilled14% 5% 3% 2% 2% 1% 1% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

94.90 100.62 101.99 102.66 103.09 103.50 103.78 103.92


renewables56% 62% 63% 64% 64% 65% 65% 65%

Scenario 9 combined the installed capacities of solar and wind from scenarios 2 and 6.

For both these scenarios, adding 200GWh only made around a 5% reduction in CCGT

use. In the combined system the first 200GWh makes a 23% reduction. Figure 39

shows the decrease in CCGT use compared to having no energy store for scenarios 2,

6 and 9.

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Having a mix of renewables seems to enhance the value of storage, and also reduce

how quickly the value of increased levels of storage diminishes. A lot of this will be

because there is more installed capacity in the combined system, and so more energy

available to be stored. However it is also enhanced by the fact that generally speaking

the need for energy storage for solar power occurs at a different time seasonally than

when excess wind power needs to be stored.

Figure 40 Curtailed Wind Vs Curtailed

Figure 39 Decrease in CCGT Use at Different Wind Capacities

72

Figure 40 shows that all spilled solar happens in the summer and most spilled wind

energy occurs in the winter months. This means the PHS system will be in use most

of the year, and as such will be helping to reduce CCGT use in both summer and

winter.

4.4. Scenario 10

Scenario 10 was intended to show the value of storage on a system with a strong mix

of renewables and nuclear that would hopefully reduce reliance on CCGT.



Factor increase to

2014's capacity:4 2.17 3 1 1 0


CCGT Max Capacity

Needed GW33 33 33 33 33 33 33 33


GWh59212 49879 48167 47594 47185 46893 46893 46893


CCGT use from no

storage

0.00% 15.76% 18.65% 19.62% 20.31% 20.81% 20.81% 20.81%

CCGT Operation

Reduction from 201431.38% 42.20% 44.18% 44.84% 45.32% 45.66% 45.66% 45.66%


Spilled10% 2% 1% 1% 0% 0% 0% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

99.41 102.66 103.25 103.45 103.60 103.70 103.70 103.70


renewables37% 40% 41% 41% 41% 41% 41% 41%

The first 200GWh in this system leads to an almost 16% reduction in the need to use

backup CCGT when compare to a system with no storage embedded in it.

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Even with no coal in the system, CCGT use has reduced by over 30% compared to the

actual 2014 figure. With additional storage this can get as high as 45%. These levels

of installed capacity are very realistic, the installed wind capacity is only 26% higher

than the UK’s capacity will be once all planned offshore wind is completed. The

installed solar capacity is equivalent to 8.5 million homes having a 3.5kW system

installed. This would save almost 100MtCO2e, which would represent around an 83%

cut in carbon emissions from electricity generation.

For storage, the first 200GWh will lead to an 80% reduction in spilled energy, but past

200GWh the levels of renewable installed would mean additional storage would be

under utilised and so uneconomical.

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4.5. Scenario 11



Factor increase to

2014's capacity:3.3 1 3 1 3 0.8


CCGT Max Capacity

Needed GW27 27 27 27 27 27 27 27


GWh49197 42690 41319 40544 40000 39722 39524 39524


CCGT use from no

storage

0.00% 13.23% 16.01% 17.59% 18.69% 19.26% 19.66% 19.66%

CCGT Operation

Reduction from 201442.99% 50.53% 52.12% 53.02% 53.65% 53.97% 54.20% 54.20%


Spilled8.05% 2.61% 1.48% 0.84% 0.39% 0.16% 0.00% 0.00%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

30.90 33.16 33.64 33.90 34.09 34.19 34.26 34.26


renewables38.18% 40.44% 40.91% 41.18% 41.36% 41.46% 41.53% 41.53%

Scenario 11 shows that once all offshore windfarms have been built, and if solar

continues to increase to the point an additional 8 million people have a 3.5kW system,

then without storage 8% of renewables will have to be curtailed. With the addition of

just 100GWh of storage, curtailed renewables will fall to 3% and with 200GWh it

would fall to 1%. The maximum capacity of backup CCGT need is 27GW, which is

the close to the value as it currently stands. 8% of renewables spilled for scenario 11

equates to 10,000 GWh. This is the equivalent of the 400MW gas turbine at Corby

running nonstop for just under three years, or the equivalent of over 3% of the total

demand in 2014 being curtailed.

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4.6. Scenario 12



Factor increase to

2014's capacity:10 0 5.25 2.722 5 0


CCGT Max Capacity

Needed GW40 40 40 40 40 40 40 40


GWh57560 42780 36961 33452 30765 28429 26730 25326


CCGT use from no storage0.00% 25.68% 35.79% 41.88% 46.55% 50.61% 53.56% 56.00%

CCGT Operation Reduction

from 201433.30% 50.42% 57.17% 61.23% 64.35% 67.05% 69.02% 70.65%


Spilled21% 17% 15% 14% 13% 12% 11% 11%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

99.99 105.13 107.15 108.37 109.30 110.12 110.71 111.20


renewables81% 86% 88% 89% 90% 91% 91% 92%

At these levels of installed renewable capacity, the value of energy storage becomes

apparent. No storage embedded means only a reduction in use of CCGT’s by a third,

but just 200GWh reduces this to almost half. 1400GWh, which is in all likelihood a

figure the UK will never build, would lead to a 70% reduction in the use of gas

turbines. At this point, 92% of electricity would be from renewables with no need for

coal or nuclear to provide baseload. 40GW of backup CCGT capacity is still required

though, purely because there is only biomass supplying some baseload and so when

the wind stops, the gas turbines have to make up for around 130GW of wind.

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4.7. Scenarios 13-15

Table 7 Scenarios 13-15 Results

Scenario 13Wind Nuclear Solar Hydro Biomass Coal Storage Required GWh

Factor increase to

2014's capacity:1 4.7 0 1 0 0 18500


Factor increase to

2014's capacity:5.2 2.17 2 2.722 5 0 9100


Factor increase to

2014's capacity:9.7 0 9 2.722 0 0 15000

The above table shows the best configurations that could be found that would both

minimise storage levels and also installed generating capacity, such that supply could

always meet demand without the need for gas turbines to be used to either load follow

or as reserve capacity.

The model was slightly adapted for these scenarios as it assumed that the PHS would

begin the year with 5000GWh, so it would be more likely to simulate what a real

energy store would look like at the beginning of the year. Also for scenario 13, energy

was not allowed to be curtailed as unlike a wind farm or solar panel, a nuclear power

station cannot simply be switched off.

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As you can see from Figure 41 nuclear alone needs the most storage to successfully

load follow. This is because nuclear goes mostly at a flat rate all year, so must

perform at a median output between minimum summer demand and maximum winter

demand, and all the extra energy it stores in summer must be kept until the winter.

18000GWh is the equivalent of over four hundred 40GWh Coire Glas PHS sites (it

must be noted Nuclear has some ability to throttle output but it hampers overall

efficiency). The renewable only option is not much better, as the grid needs a lot of

PHS to store up renewable power throughout the year so that it can be supplied almost

entirely from pumped storage for the weeks in winter when high pressure systems

hang over the UK. The lowest of the storage levels is a mix of the two, but still at

9100GWh would be, although technically feasible given the geographical nature of

Scotland and Wales, politically and environmentally impossible to build.

Figure 41 Storage Required To Replace CCGT Use

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5. Discussion

5.1. When Will Curtailment Start and Storage be Required?

The first thing to be said is that curtailment of renewable energy already happens to

some degree within the national grid. Whitelee Wind Farm is curtailed at times

throughout the year due to the fact it can only export its energy to the central belt as

there is a bottleneck in the transmission lines down to population centres in the south,

although the new western HVDC link hopes to alleviate this (GOV.UK, 2012). If the

transmission infrastructure is reinforced adequately, then spillage will only be due to

lack of storage and not because of an inability to move the power to where it is either

neededor can be stored.

Curtailment will mean a few things to the renewable sector, as installed capacity gets

higher spillage will increase and thus the value economically of adding additional

capacity lessons this will likely disencentivise the large scale installation of

renewables the UK needs to decarbonioze its electricty network. It also means more

turbines will have to be installed to be able to do the job of less turbine plus storage

wich will have ramifications in terms of land use. So after a certain point building

additional storage will perhaps become econmoical and acceptable in terms of land

use.

Analysis of the various scenarios shows that need for energy storage depends entirely

on the make-up of the grid at the time. Scenario 11 is probably closest in appearance

to what the national grid will look like in the next ten years. Even with no increase in

79

installed solar capacity or biomass prescribed by scenario 11 around 6% of renewable

energy would have to be spilled without storage.

From looking at the model, spillage seems to begin to rapidly increase at the mark

where around 30% of total energy is from renewable sources mainly made up of wind

capacity. Biomass and hydro due to their fairly consistent output do not lead to

markedly increased spillage, solar only begins to contribute to spillage once additional

installed capacity reaches 20GW. In all likelihood once the competition of all planned

onshore and offshore wind is completed and if coal use is not reduced in favour of the

more dynamic CCGT’s then we could see curtailment of renewables get as high as

10%

One added detail is that all simulations assume a 60/40 onshore to offshore split, but

in fact what is currently planned is around 27GW of new offshore with new onshore

being hampered by cuts in subsidies. The split will therefore be more likely to

resemble a 75/25 ratio in favour of offshore. This means any simulations that have

increased capacities of wind will be more energetic than what is modelled in the

scenarios, and the need for storage will be enhanced further.

Given the rapid uptake in solar seen in 2014 and with the price of solar continuing to

fall along with around 27GW of offshore wind that will in all likelihood be completed

in the next five to ten years, we could expect to see significant increases in

curtailment of renewable energy by 2025.

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5.2. How Much PHS Could be Built in the UK?

The European Strategic Energy Technology (SET), which is part of the European

Commission, funded a GIS appraisal of potential PHS across Europe. Its findings

showed that the UK had a realisable potential storage capacity of over 9000GWh

(Gimeno-Gutiérrez, 2013). The Geographic Information Systems (GIS) aspects of the

appraisal included only existing grid infrastructure, so the potential storage for the UK

will likely be higher still once the Beauly-Denny transmission line is completed,

which would open up the possibility of new possible PHS sites in the Highlands.

Given this and the scenarios investigated which showed diminishing returns at

increasing amounts of storage, the UK has as much potential PHS as it requires.

5.3. What Scenario is best?

There are numerous different types of configuration the UK could have in its balance

of generating mix in the future and the scenarios selected were mostly picked to

investigate what role additional storage could play with various mixes, no one

scenario is any better than another, even the scenarios that involved 10 times the

current capacity of renewables are technically achievable and only rely on political

will for them to be implemented.

In all likelihood what we can say is that new nuclear plants will be built and they will

be a higher capacity than the ones they are replacing and that all rounds of planned

offshore currently planned will be built. The price of solar has continued to fall and

even without subsidies they will be economical for those living in the South of the

UK. A Final scenario was designed to take into account these factors and see how a

81

realistic level of pumped hydro storage could help eliminate further reliance on gas

turbines.

This final scenario involved an installed capacity of around 85GW of wind, 40GW of

solar, 20Gw of nuclear power, hydro power was only increased marginally due to the

lack of suitable sites and a 3 times increase in biomass was put in as over this the

ability to source fuel sustainably would in all likelihood become an issue. None of

these figures are very large and in fact given that the majority of the wind farms in the

future will probably be offshore with higher capacity factors 75GW of installed wind

will do the same job as what the model simulates 85GW will output. 40GW of solar

involves just under 12 million homes having a 3.5kW system so definitely not outwith

the realms of possibility of what we could see in the next 10-20 years.

Table 8 Final Scenario Results

FINAL SCENARIO

7 2.17 4 1.2 3 0


CCGT Max Capacity

Needed GW30 30 30 30 30 30 30 30


GWh25311 16451 13481 11755 10412 9167 8226 7536


CCGT use from no storageN/A 35.01% 46.74% 53.56% 58.86% 63.78% 67.50% 70.22%


from 201470.67% 80.94% 84.38% 86.38% 87.93% 89.38% 90.47% 91.27%


Spilled31.57% 27.47% 26.06% 25.21% 24.54% 23.91% 23.44% 23.08%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

111.20 114.28 115.31 115.91 116.38 116.81 117.14 117.38


renewables49% 51% 52% 53% 54% 54% 54% 55%

With no storage in this system decrease in CCGT use is still 70% of the 2014 value

but with the addition of 200GWh this could increase to around 85%, spillage would

82

still be reasonably high but at this point CCGT’s would only be in operation in times

when high pressure systems where over the UK for longer periods of time. In terms of

carbon emissions they would decrease from 2014’s value of around 121 MtCO2e to

less than 10 MtCO2e, a decrease in carbon emissions from the electrical supply of

almost 92%. This is felt to be the best scenario that has a balance of what can be

realistically implemented but also have radical cuts in carbon emissions.

5.4. How Much PHS Should be Built?

The amount of useful storage required by the grid depends entirely on how much

renewables make up the total generating capacity. However from all scenarios

investigated, there seems to be a very strong case for the building of 100-200GWh of

new PHS, in all scenarios the addition of 200GWh led to a drastic reduction in

curtailment of renewable energy and consequently reliance on CCGT’s. If there are

plans to build much more wind farms beyond those planned, or the capacity factors

out at sea are higher than previously thought, then there could be a case for a further

additional 200GWh to minimise spillage. However, beyond this the advantages of

additional storage quickly diminish in terms of the economics of having large dams sit

seldom used for long periods of time versus relying on gas turbines to get through the

brief periods when renewables will effectively generate zero power.

5.5. Infrastructure Requirements to Store Surplus Renewable Energy

If the UK decided to build the recommended 200GWh of storage in all likelihood the

majority of the sites would be located in the highlands of Scotland, unfortunately the

Grid Infrastructure in place in these locations will be unlikely to handle the size of

83

power flows involved. The largest of planned offshore windfarms Doggerbank will be

around 5GW alone, this is more than a third of current installed capacity of wind and

will likely connect to the mainland at Redcar and Cleveland in Teeside

(RenwableEnergyMagazine, 2015). The existing transmission infrastructure between

there and the Highlands has previously been shown in Figure 2.3.4 on Page 34, the

majority of the distance is covered in 275kV lines that would increase electrical losses

when compared to the 400kV lines the cover much of the South. Beauly Denny the

current new transmission line running from the highlands to the central belt will help

to relive some of the stresses that would inevitably be placed on the existing

transmission lines handling such large power flows but in all likelihood additional

lines will have to be put in place if all the renewable energy generated from the solar

panels and offshore wind of England and Wales wishes to be stored in Scottish

pumped hydro.

Luckily OFGEM have recently fast tracked £4 billion to improve transmission

infrastructure in the Highlands and Islands so there is hope that by the time the need

for storage arises then the national grid will have the infrastructure in place to move

such large sums of energy to where it can be stored (HIE, 2015).

In terms of Policy for increased PHS to be viable then there must be an end to

OFGEM’s current system of transmission charges being based upon distance as this

severely disadvantages the movement of renewable energy from the places it can be

generated to the places it can be stored and then on to where there is demand for it as

this will involve much larger round trips than we currently see in conventional fossil

fuel generation.

84

5.6. What This Means for CCGT’s Role in the Grid

As the scenarios show, storage levels do not help reduce much the size of reserve

capacity required to back-up the renewable-PHS system, as it requires massive

amounts of both renewables and storage to see the UK through a high pressure system

that lasts weeks. If the UK wishes to get rid of its coal power station and allow its

nuclear power to be slowly decommissioned, then an additional 15GW of CCGT

capacity would be required regardless of the amount of renewables installed.

There is a way reserve capacity could be reduced without adding lots of additional

storage and that would be by running the CCGT’s at the same time as the pumped

hydro storage such that both the output power of the PHS and CCGT was reduced but

each ran for longer, so instead of for example running 40GW of PHS for three days

then 40GW of CGGT for 3 days you would run 20GW each simultaneously for 6

days.

This has one big disadvantage as your pumped hydro store could still run dry before

the renewable generation begins to meet supply but you do not have any remaining

spare capacity to meet demand, if this were to happen there would be massive

blackouts and a probable total collapse of the national grid. The generally

unpredictable nature of renewables would in all likelihood mean it would be better to

utilize PHS energy stores for as long as possible and then move onto backup CCGT

such that use of the previously stored renewable energy was always maximised, this

has the unfortunate side effect of increasing the size of the reserve capacity needed.

85

6. In Conclusion

From the results the author would recommend that in around the next 10 years

between 100-200GWh of new pumped hydro storage should be built purely to prevent

the curtailment of renewable energy and reduce the use of CCGT’s as a load

following mechanism. Unfortunately this will also mean the adding of more CCGT

capacity to act in reserve for when renewable output is negligible and the pumped

hydro stores are empty, even if overall CCGT use will be far lower than as it currently

stands.

To facilitate this the national grid will need to be strengthened such that large power

flows may operate from England to Scotland and back again.

7. Further Work

If time or data available had permitted the following would have been added to the

model:

Offshore and Onshore wind capacity increases would have been spate to take

into account their different capacity factors.

An option for changing the demand side characteristics throughout the year

would be added to take into account the increased focus seen in recent years

on demand management.

Wave and Tidal power would be added in to the model to complete the

possible mix of renewables that can be expected to be seen in the future.

Work out the required improvements to the transmission infrastructure if the

various scenarios were implemented.

86

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90

9. Appendix

Table 9 Appendix: Scenario 1 Results


Factor increase to

2014's capacity:3 1 0 1 1 0

37GW


CCGT Max Capacity

Needed GW41 41 41 41 41 41 41 41


GWh166287 166287 166287 166287 166287 166287 166287 166287


CCGT use from no

storage

0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%

CCGT Operation

Reduction from 2014-92.71% -92.71% -92.71% -92.71% -92.71% -92.71% -92.71% -92.71%


Spilled0% 0% 0% 0% 0% 0% 0% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

62.19 62.19 62.19 62.19 62.19 62.19 62.19 62.19


renewables25% 25% 25% 25% 25% 25% 25% 25%

91



Factor increase to

2014's capacity:6 1 0 1 1 0

75GW


CCGT Max Capacity

Needed GW40 40 40 40 40 40 40 40


GWh111178 105294 104093 103695 103497 103312 103312 103312




from 2014-28.84% -22.02% -20.63% -20.17% -19.94% -19.73% -19.73% -19.73%


Spilled6% 1% 1% 0% 0% 0% 0% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

81.35 83.39 83.81 83.95 84.02 84.08 84.08 84.08


renewables43% 45% 46% 46% 46% 46% 46% 46%

92



Factor increase to

2014's capacity:9 1 0 1 1 0

112GW


CCGT Max Capacity

Needed GW40 40 40 40 40 40 40 40


GWh79366 69594 65474 62320 60118 58373 56861 55722


CCGT use from no

storage

0.00% 12.31% 17.50% 21.48% 24.25% 26.45% 28.36% 29.79%

CCGT Operation

Reduction from 20148.03% 19.35% 24.12% 27.78% 30.33% 32.35% 34.11% 35.43%


Spilled20% 15% 13% 11% 10% 9% 8% 8%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

92.41 95.80 97.24 98.33 99.10 99.71 100.23 100.63


renewables54% 57% 59% 60% 60% 61% 61% 62%

93



Factor increase to

2014's capacity:12 1 0 1 1 0

150GW


CCGT Max Capacity

Needed GW39 39 39 39 39 39 39 39


GWh60314 48699 42559 38740 36263 34459 32928 31574




from 201430.10% 43.56% 50.68% 55.10% 57.98% 60.07% 61.84% 63.41%


Spilled32% 27% 25% 23% 22% 22% 21% 20%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

99.03 103.07 105.20 106.53 107.39 108.02 108.55 109.02


renewables60.1% 64.0% 66.1% 67.5% 68.4% 69.0% 69.6% 70.1%

94



Factor increase to

2014's capacity:1 0 3 1 1 0


CCGT Max Capacity

Needed GW42 42 42 42 42 42 42 42


GWh179494 179488 179488 179488 179488 179488 179488 179488


CCGT use from no

storage

0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%

CCGT Operation

Reduction from 2014-108.01% -108.00% -108.00% -108.00% -108.00% -108.00% -108.00% -108.00%


Spilled0% 0% 0% 0% 0% 0% 0% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

57.60 57.60 57.60 57.60 57.60 57.60 57.60 57.60


renewables20% 20% 120% 220% 320% 420% 520% 620%

95



Factor increase to

2014's capacity:1 0 6 1 1 0


CCGT Max Capacity

Needed GW42 42 42 42 42 42 42 42


GWh155439 151118 151118 151118 151118 151118 151118 151118




from 2014-80.13% -75.13% -75.13% -75.13% -75.13% -75.13% -75.13% -75.13%


Spilled5% 0% 0% 0% 0% 0% 0% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

65.96 67.46 67.46 67.46 67.46 67.46 67.46 67.46


renewables28% 30% 30% 30% 30% 30% 30% 30%

96



90GW Solar Capacity

Factor increase to

2014's capacity:1 0 9 1 1 0


CCGT Max Capacity

Needed GW42 42 42 42 42 42 42 42


GWh141446 123028 122621 122621 122621 122621 122621 122621


CCGT use from no

storage

0.00% 13.02% 13.31% 13.31% 13.31% 13.31% 13.31% 13.31%

CCGT Operation

Reduction from 2014-63.92% -42.57% -42.10% -42.10% -42.10% -42.10% -42.10% -42.10%


Spilled17% 0% 0% 0% 0% 0% 0% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

70.82 77.23 77.37 77.37 77.37 77.37 77.37 77.37


renewables33% 40% 40% 40% 40% 40% 40% 40%

97



120GW Solar Capacity

Factor increase to

2014's capacity:1 0 12 1 1 0


CCGT Max Capacity

Needed GW42 42 42 42 42 42 42 42


GWh132904 101509 97674 97095 96851 96648 96444 96251




from 2014-54.02% -17.64% -13.19% -12.52% -12.24% -12.00% -11.77% -11.54%


Spilled27% 6% 3% 3% 3% 2% 2% 2%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

73.79 84.71 86.04 86.24 86.33 86.40 86.47 86.54


renewables36% 46% 48% 48% 48% 48% 48% 48%

98



Factor increase to

2014's capacity:6 1 6 1 1 0


CCGT Max Capacity

Needed GW40 40 40 40 40 40 40 40


GWh72203 55753 51816 49863 48647 47473 46645 46244


CCGT use from no

storage

0.00% 22.78% 28.24% 30.94% 32.63% 34.25% 35.40% 35.95%

CCGT Operation

Reduction from 201416.33% 35.39% 39.95% 42.21% 43.62% 44.99% 45.94% 46.41%


Spilled14% 5% 3% 2% 2% 1% 1% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

94.90 100.62 101.99 102.66 103.09 103.50 103.78 103.92


renewables56% 62% 63% 64% 64% 65% 65% 65%

99



Factor increase to

2014's capacity:4 2.17 3 1 1 0


CCGT Max Capacity

Needed GW33 33 33 33 33 33 33 33


GWh59212 49879 48167 47594 47185 46893 46893 46893


CCGT use from no

storage

0.00% 15.76% 18.65% 19.62% 20.31% 20.81% 20.81% 20.81%

CCGT Operation

Reduction from 201431.38% 42.20% 44.18% 44.84% 45.32% 45.66% 45.66% 45.66%


Spilled10% 2% 1% 1% 0% 0% 0% 0%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

99.41 102.66 103.25 103.45 103.60 103.70 103.70 103.70


renewables37% 40% 41% 41% 41% 41% 41% 41%

100



Factor increase to

2014's capacity:3.3 1 3 1 3 0.8


CCGT Max Capacity

Needed GW27 27 27 27 27 27 27 27


GWh49197 42690 41319 40544 40000 39722 39524 39524


CCGT use from no

storage

0.00% 13.23% 16.01% 17.59% 18.69% 19.26% 19.66% 19.66%

CCGT Operation

Reduction from 201442.99% 50.53% 52.12% 53.02% 53.65% 53.97% 54.20% 54.20%


Spilled8.05% 2.61% 1.48% 0.84% 0.39% 0.16% 0.00% 0.00%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

30.90 33.16 33.64 33.90 34.09 34.19 34.26 34.26


renewables38.18% 40.44% 40.91% 41.18% 41.36% 41.46% 41.53% 41.53%

101



Factor increase to

2014's capacity:10 0 5.25 2.722 5 0


CCGT Max Capacity

Needed GW40 40 40 40 40 40 40 40


GWh57560 42780 36961 33452 30765 28429 26730 25326




from 201433.30% 50.42% 57.17% 61.23% 64.35% 67.05% 69.02% 70.65%


Spilled21% 17% 15% 14% 13% 12% 11% 11%

Total CO2 Saving

compared to 2014

Emmisions (MtCO2e)

99.99 105.13 107.15 108.37 109.30 110.12 110.71 111.20


renewables81% 86% 88% 89% 90% 91% 91% 92%

Table 21 Appendix: Scenarios 13-15 Results


Factor increase to

2014's capacity:1 4.7 0 1 0 0 18500


Factor increase to

2014's capacity:5.2 2.17 2 2.722 5 0 9100


Factor increase to

2014's capacity:9.7 0 9 2.722 0 0 15000

The Role of Additional Pumped Hydro Storage in a …Department of Mechanical and Aerospace Engineering The Role of Additional Pumped Hydro Storage in a Low Carbon UK Grid Author: Thomas

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